Several species of nudibranch mollusc considered rare, can be found at Mumbles, Swansea, UK. Collections were made during every spring low tide, weather permitting, for three years. During the course of this investigation several aspects of nudibranch ecology were investigated. Oogenesis and spermatogenesis were studied using light microscopy in Thecacera pennigera (Montagu), Ancula gibbosa (Risso), Facelina auriculata (Müller) and Palio nothus (Johnson). Significant differences were found in oocyte length in relation to animal size, and nucleolus diameter in relation to the oocyte stage of development. It was concluded that oogenesis in all four species occurs in discrete waves; this is probably attributable to the lack of space within the female follicles once vitellogenesis has occurred. Spermatogenesis was uniform among T. pennigera, F. auriculata and A. gibbosa. However, a difference in the orientations of the developing spermatids was observed in P. nothus. Data presented in this chapter for P. nothus contradict previous studies investigating other Palio spp. Veliger development was investigated using T. pennigera, P. nothus, Onchidoris bilamellata (L.) A. gibbosa, and Cuthona gymnota Couthouy. No veliger development or shell growth were observed in A. gibbosa or C. gymnota. Small growth was observed in P. nothus, but the veligers did not structurally develop further towards metamorphic competency. O. bilamellata and T. pennigera showed significant development; both exhibited shell growth and developed eye spots. The latter species also developed a larval heart and underwent mantle fold retraction. However, when introduced to Bugula plumose (Pallas) T. pennigera failed to undergo metamorphosis.